Inspection Method and Apparatus, Lithographic Apparatus, Lithographic Processing Cell and Device Manufacturing Method

ABSTRACT

A device manufacturing method is disclosed. A radiated spot is directed onto a target pattern formed on a substrate. The radiated spot is moved along the target pattern in a series of discrete steps, each discrete step corresponding to respective positions of the radiated spot on the target pattern, Measurement signals are generated that correspond to respective ones of the positions of the radiated spot on the target pattern. A single value is determined that is based on the measurement signals and that is representative of the property of the substrate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation U.S. patent application Ser. No.16/168,355, filed on Oct. 23, 2018, which is a continuation of U.S.patent application No. 15,913,253, filed on Mar. 6, 2018, which is adivisional of U.S. patent application Ser. No. 14/536,979, filed on Nov.10, 2014, which is a continuation of U.S. patent application Ser. No.12/867,416, 371(c) Date: Feb. 18, 2011, which is a U.S. National StageEntry of int′) Application No. PCT/EP2009/001141, filed on Feb. 18,2009, which claims priority from U.S. Provisional application61/064,312, filed on Feb. 27, 2008, which are all incorporated herein intheir entirety by reference.

BACKGROUND Field of Invention

The present invention relates to methods of inspection usable, forexample, in the manufacture of devices by lithographic techniques and tomethods of manufacturing devices using lithographic techniques.

Background

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate.

In order to monitor the lithographic process, it is desitable to measureparameters of the patterned substrate, for example the overlay errorbetween successive layers formed in or on it. This measurement may takeplace during the lithographic process, or separately, from it, but isusually carried out using a separate metrology apparatus from thelithographic apparatus.

There are various techniques for making measurements of the microscopicstructures formed in lithographic processes, including the use ofscanning electron microscopes and various specialized tools. One form ofspecialized inspection tool is a scatterometer in which a beam ofradiation is directed onto a target on the surface of the substrate andproperties of the scattered or reflected beam are measured. By comparingthe properties of the beam before and after it has been reflected orscattered by the substrate, the properties of the substrate can bedetermined. This can be done, for example, by comparing the reflectedbeam with data stored in a library of known measurements associated withknown substrate properties. Two main types of scatterometer are known.Spectroscopic scatterometers direct a broadband radiation beam onto thesubstrate and measure the spectrum (intensity as a function ofwavelength) of the radiation scattered into a particular narrow angularrange. Angularly resolved scatterometers direct a monochromaticradiation beam onto the substrate, the intensity of the scatteredradiation being measured as a function of angle.

The scatterometer may be adapted to measure the overlay of targets inthe form of two misaligned gratings or periodic structures by measuringasymmetry in the reflected spectrum and/or the detection configuration,the asymmetry being related to the extent of the overlay. In order touse smaller overlay targets, which may be positioned on the scribe lanesbetween products or on the product itself, it is known to use smallmeasurement spots, that is the area of the incident radiation on thetarget. However, the use of such small measurement spots means thatalthough the spots may fill small targets, averagely sized targets orlarge targets will be under-filled by the measurement spot such that fora grating imperfections in each grating line will affect the measuredangle-resolved spectrum more strongly than in the case that all thegrating lines contribute.

US 2002/0135783 discloses a method and apparatus for evaluating periodicstructures formed on a sample in which a probe beam is continuouslyscanned over a wafer until sufficient data can be measured. However suchan arrangement produces a lower signal to noise ratio than with a largermeasurement spot.

U.S. Pat. No. 6,023,338 discloses a method of determining offset betweenadjacent layers of a semiconductor device in which a measurement spotscans across the gratings on a wafer.

SUMMARY

It is desirable to provide a scatterometer and a method for measuring aproperty of a substrate in which a smaller measurement spot may be usedwithout reducing the signal to noise ratio achievable with a largermeasurement spot.

According to an embodiment, a lithographic apparatus includes ascatterometer configured to measure a property of the substrate. Thescatterometer includes a radiation source configured to produce aradiated spot on a target on the substrate, where the radiated spotincludes positions on the target. The scatterometer further includes adetector configured to generate measurement signals that correspond torespective ones of the positions of the radiated spot and a processorconfigured to output, based on the measurement signals, a single valuethat is representative of the property of the substrate.

According to another embodiment, a lithographic cell includes a coaterconfigured to coat a substrate with a radiation sensitive layer, alithographic apparatus configured to expose images onto the radiationsensitive layer of the substrate, a developer configured to developimages exposed by the lithographic apparatus, and a scatterometerconfigured to measure a property of the substrate. The scatterometerincludes a radiation source configured to produce a radiated spot on atarget on the substrate, where the radiated spot includes positions onthe target. The scatterometer further includes a detector configured togenerate measurement signals that correspond to respective ones of thepositions of the radiated spot and a processor configured to output,based on the measurement signals, a single value that is representativeof the property of the substrate.

Yet in another embodiment, a device manufacturing method is provided.The method includes directing a radiated spot onto a target patternformed on a substrate and moving the radiated spot along the targetpattern in a series of discrete steps, where each discrete stepcorresponds to respective positions of the radiated spot on the targetpattern. The method further includes generating measurement signals thatcorrespond to respective ones of the positions of the radiated spot onthe target pattern and determining, based on the measurement signals, asingle value that is representative of the property of the substrate.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus in accordance with an embodimentof the invention;

FIG. 2 depicts a lithographic cell or cluster in accordance with anembodiment of the invention;

FIG. 3 depicts a first scatterometer in accordance with an embodiment ofthe invention;

FIG. 4 depicts a second scatterometer in accordance with an embodimentof the invention;

FIG. 5 illustrates a measurement spot incident on a grating;

FIG. 6 illustrates the movement of the measurement spot over the gratingin accordance with an embodiment of the invention;

FIG. 7 illustrates a first data processing technique in accordance withan embodiment of the invention; and

FIG. 8 illustrates a second data processing technique in accordance withan embodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus LA. The apparatusincludes an illumination system (illuminator) IL configured to conditiona radiation beam B (e.g. UV radiation or DUV radiation); a patterningdevice support or support structure (e.g. a mask table) MT constructedto support a patterning device (e.g. a mask) MA and connected to a firstpositioner PM configured to accurately position the patterning device inaccordance with certain parameters; a substrate table (e.g. a wafertable) WI constructed to hold a substrate (e.g. a resist-coated wafer) Wand connected to a second positioner PW configured to accuratelyposition the substrate in accordance with certain parameters; and aprojection system (e.g. a refractive projection lens system) PLconfigured to project a pattern imparted to the radiation beam B bypatterning device MA onto a target portion C (e.g. including one or moredies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The patterning device support holds the patterning device in a mannerthat depends on the orientation of the patterning device; the design ofthe lithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The patterning device support can use mechanical, vacuum, electrostaticor other clamping techniques to hold the patterning device. Thepatterning device support may be a frame or a table, for example, whichmay be fixed or movable as required. The patterning device support mayensure that the patterning device is at a desired position, for examplewith respect to the projection system. Any use of the terms “reticle” or“mask” herein may be considered synonymous with the more general term“patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam, which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables), hi such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein at least aportion of the substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDincluding, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may include an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL mayinclude various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., mask)MA, which is held on the patterning device support (e.g., mask table)MT, and is patterned by the patterning device. Having traversed thepatterning device (e.g. mask) MA, the radiation beam B passes throughthe projection system PL, which focuses the beam onto a target portion Cof the substrate W. With the aid of the second positioner PW andposition sensor IF (e.g. an interferometric device, linear encoder, 2-Dencoder or capacitive sensor), the substrate table WT can be movedaccurately, e.g. so as to position different target portions C in thepath of the radiation beam B. Similarly, the first positioner PM andanother position sensor (which is not explicitly depicted in FIG. 1) canbe used to accurately position the patterning device (e.g. mask) MA withrespect to the path of the radiation beam B, e.g. after mechanicalretrieval from a mask library, or during a scan. In general, movement ofthe patterning device (e.g. mask table) MT may be realized with the aidof a long-stroke module (coarse positioning) and a short-stroke module(fine positioning), which form part of the first positioner PM.Similarly, movement of the substrate table WT may be realized using along-stroke module and a short-stroke module, which form part of thesecond positioner PW. hi the case of a stepper (as opposed to a scanner)the patterning device support (e.g. mask table) MT may be connected to ashort-stroke actuator only, or may be fixed. Patterning device (e.g.mask) MA and substrate W may be aligned using mask alignment marks M1,M2 and substrate alignment marks P1, P2. Although the substratealignment marks as illustrated occupy dedicated target portions, theymay be located in spaces between target portions (these are known asscribe-lane alignment marks). Similarly, in situations in which morethan one die is provided on the patterning device (e.g. mask) MA, themask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

1. In step mode, the patterning device support (e.g. mask table) MT andthe substrate table WT are kept essentially stationary, while an entirepattern imparted to the radiation beam is projected onto a targetportion C at one time (i.e. a single static exposure). The substratetable WT is then shifted in the X and/or Y direction so that a differenttarget portion C can be exposed. In step mode, the maximum size of theexposure field limits the size of the target portion C imaged in asingle static exposure.

2. In scan mode, the patterning device support (e.g. mask table) MT andthe substrate table WT are scanned synchronously while a patternimparted to the radiation beam is projected onto a target portion C(i.e. a single dynamic exposure). The velocity and direction of thesubstrate table WT relative to the patterning device support (e.g. masktable) MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the patterning device support (e.g. mask table) MTis kept essentially stationary holding a programmable patterning device,and the substrate table WT is moved or scanned while a pattern impartedto the radiation beam is projected onto a target portion C. In thismode, generally a pulsed radiation source is employed and theprogrammable patterning device is updated as required after eachmovement of the substrate table WT or in between successive radiationpulses during a scan. This mode of operation can be readily applied tomaskless lithography that utilizes programmable patterning device, suchas a programmable mirror array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

As shown in FIG. 2, the lithographic apparatus LA forms part of alithographic cell LC, also sometimes referred to a lithocell or cluster,which also includes apparatus to perform pre- and post-exposureprocesses on a substrate. Conventionally these include spin coaters SCto deposit resist layers, developers DE to develop exposed resist, chillplates CH and bake plates BK. A substrate handler, or robot, RO picks upsubstrates from input/output ports I/O1, I/O2, moves them between thedifferent process apparatus and delivers then to the loading bay LB ofthe lithographic apparatus. These devices, which are often collectivelyreferred to as the track, are under the control of a track control unitTCU which is itself controlled by the supervisory control system SCS,which also controls the lithographic apparatus via lithography controlunit LACU. Thus, the different apparatus can be operated to maximizethroughput and processing efficiency.

In order that the substrates that are exposed by the lithographicapparatus are exposed correctly and consistently, it is desirable toinspect exposed substrates to measure properties such as overlay errorsbetween subsequent layers, line thicknesses, critical dimensions (CD),etc. If errors are detected, adjustments may be made to exposures ofsubsequent substrates, especially if the inspection can be done soon andfast enough that other substrates of the same batch are still to beexposed. Also, already exposed substrates may be stripped andreworked—to improve yield—or discarded—thereby avoiding performingexposures on substrates that are known to be faulty. In a case whereonly some target portions of a substrate are faulty, further exposurescan be performed only on those target portions which are good.

An inspection apparatus is used to determine the properties of thesubstrates, and in particular, how the properties of differentsubstrates or different layers of the same substrate vary from layer tolayer. The inspection apparatus may be integrated into the lithographicapparatus LA or the lithocell LC or may be a stand-alone device. Toenable most rapid measurements, it is desirable that the inspectionapparatus measure properties in the exposed resist layer immediatelyafter the exposure. However, the latent image in the resist has a verylow contrast—there is only a very small difference in refractive indexbetween the parts of the resist which have been exposed to radiation andthose which have not and not all inspection apparatus have sufficientsensitivity to make useful measurements of the latent image. Thereforemeasurements may be taken after the post-exposure bake step (PEB) whichis customarily the first step carried out on exposed substrates andincreases the contrast between exposed and unexposed parts of theresist. At this stage, the image in the resist may be referred to assemi-latent. It is also possible to make measurements of the developedresist image—at which point either the exposed or unexposed parts of theresist have been removed—or after a pattern transfer step such asetching. The latter possibility limits the possibilities for rework offaulty substrates but may still provide useful information.

FIG. 3 depicts a scatterometer SMI which may be used in an embodiment ofthe present invention. It includes a broadband (white light) radiationprojector 2 which projects radiation onto a substrate W. The reflectedradiation is passed to a spectrometer detector 4, which measures aspectrum 10 (intensity as a function of wavelength) of the specularreflected radiation. From this data, the structure or profile givingrise to the detected spectrum may be reconstructed by processing unitPU, e.g. by Rigorous Coupled Wave Analysis and non-linear regression orby comparison with a library of simulated spectra as shown at the bottomof FIG. 3. The processing unit may be termed hereinafter as the“processor”. In general, for the reconstruction the general form of thestructure is known and some parameters are assumed from knowledge of theprocess by which the structure was made, leaving only a few parametersof the structure to be determined from the scatterometry data. Such ascatterometer may be configured as a normal-incidence scatterometer oran oblique-incidence scatterometer.

Another scatterometer SM2 that may be used in an embodiment of thepresent invention is shown in FIG. 4. In this device, the radiationemitted by radiation source 2 is focused using lens system 12 throughinterference filter 13 and polarizer 17, reflected by partiallyreflective surface 16 and is focused onto substrate W via a microscopeobjective lens 15, which has a high numerical aperture (NA), preferablyat least 0.9 and more preferably at least 0.95. Immersion scatterometersmay even have lenses with numerical apertures over 1. The reflectedradiation then transmits through partially reflective surface 16 into adetector 18 in order to have the scatter spectrum detected. The detectormay be located in the back-projected pupil plane 11, which is at thefocal length of the lens system 15, however the pupil plane may insteadbe re-imaged with auxiliary optics (not shown) onto the detector. Thepupil plane is the plane in which the radial position of radiationdefines the angle of incidence and the angular position defines azimuthangle of the radiation. The detector is preferably a two-dimensionaldetector so that a two-dimensional angular scatter spectrum of asubstrate target 30 can be measured. The detector 18 may be, forexample, an array of CCD or CMOS sensors, and may use an integrationtime of, for example, 40 milliseconds per frame.

A reference beam is often used for example to measure the intensity ofthe incident radiation. To do this, when the radiation beam is incidenton the beam splitter 16 part of it is transmitted through the beamsplitter as a reference beam towards a reference mirror 14. Thereference beam is then projected onto a different part of the samedetector 18.

A set of interference filters 13 is available to select a wavelength ofinterest in the range of about 405-790 run or even lower, such as about200-300 nm. The interference filter may be tunable rather than includinga set of different filters. A grating could be used instead ofinterference filters.

The detector 18 may measure the intensity of scattered light at a singlewavelength (or narrow wavelength range), the intensity separately atmultiple wavelengths or integrated over a wavelength range. Furthermore,the detector may separately measure the intensity of transversemagnetic- and transverse electric-polarized light and/or the phasedifference between the transverse magnetic- and transverseelectric-polarized light.

Using a broadband light source (i.e. one with a wide range of lightfrequencies or wavelengths—and therefore of colors) is possible, whichgives a large etendue, allowing the mixing of multiple wavelengths. Theplurality of wavelengths in the broadband preferably each has abandwidth of δλ and a spacing of at least 2δλ (i.e. twice thebandwidth). Several “sources” of radiation can be different portions ofan extended radiation source which have been split using fiber bundles.In this way, angle resolved scatter spectra can be measured at multiplewavelengths in parallel. A 3-D spectrum (wavelength and two differentangles) can be measured, which contains more information than a 2-Dspectrum. This allows more information to be measured which increasesmetrology process robustness. This is described in more detail in EP1,628,164 A.

The target 30 on substrate W may be a grating, which is printed suchthat after development, the bars are formed of solid resist lines. Thebars may alternatively be etched into the substrate. This pattern issensitive to chromatic aberrations in the lithographic projectionapparatus, particularly the projection system PL, and illuminationsymmetry and the presence of such aberrations will manifest themselvesin a variation in the printed grating. Accordingly, the scatterometrydata of the printed gratings is used to reconstruct the gratings. Theparameters of the grating, such as line widths and shapes, may be inputto the reconstruction process, performed by processing unit PU, fromknowledge of the printing step and/or other scatterometry processes.

Further details of a scatterometer which may be used in a scatterometerin accordance with the invention are described in U.S. Ser. No.10/918,742, the contents of which are incorporated herein by reference.

Referring now also to FIGS. 5 and 6, in use of the scatterometer for theexample of the target 30 being a grating on substrate W, in accordancewith an embodiment of the invention the measurement spot on the gratingonly extends over a few periods of the grating. As indicated by spotpositions 61, 62, 63, and 64 in FIG. 6 however, in accordance with anembodiment of the invention the position of the measurement spot ismoved in discrete steps over the length of the target the spots having aportion overlap. The size of the radiation spot is smaller than thetarget in one direction along the target, the position of the radiationspot moving along the surface in the direction in a series of discretesteps. In FIG. 6, for the sake of clarity only four positions 61, 62,63, 64 of the measurement spot are depicted. It will be appreciatedhowever, that in practice there may be less, for example two, orconsiderably more than four spot positions, typically up to ten or evenmore. The detector 18 is arranged to measure the reflected light at eachposition of the spot together with, at each position of the measurementspot, a reference image produced by the reference beam split off by thebeam splitter 16 and reflected back by the reference mirror 14. Themeasurements at each spot position are then added to produce a singlesignal from the target such that the parameters of the grating, such asline width and shape, may be used in the reconstruction processperformed by the processing unit PU, using the parameters used to printthe grating.

Turning now also to FIG. 7 in a first processing technique performed bythe processing unit PU in a scatterometer in accordance with theinvention data representing the images of the measured angle resolvedspectra image (1), image (2) . . . image (n) produced from lightreflected by each position of the measurement spot are combined toproduce data representing a single image in order to perform theparameter calculations, and derive the required parameter.

Thus, in FIG. 7 in the top row of boxes data representing each image (1). . . (n) are corrected, using data representing the correspondingreference images produced by the reference beam measured on detector 18at each measurement spot position. This correction may consist of one ormore of the DC level correction, ghost image correction of ghost imagesproduced by spurious reflections for example, intensity level correctionor illumination shape correction. It will be appreciated that in somecases this correction procedure may be omitted. The result is a set ofcorrected images (1) . . . (n). These images are then combined into asingle image by, for example, taking the average or mean value of allthe images. Alternatively, the median for the images may be calculated.Parameter calculation is then performed, for example CD reconstruction,overlay calculation or focus dose calculation.

It will be appreciated that while on the conventional arrangement ofmeasuring a grating with a small spot which measures the physicalproperties of a limited number of lines only, in a scatterometer inaccordance with an embodiment the invention a better measurement of theaverage properties of the target can be obtained as information frommore grating lines is combined. It will also be appreciated that by useof such a method compared to a method in which the spot is continuouslyscanned over the target, a higher signal to noise level can be producedas the signal is no longer limited by the signal to noise level of thedetector due to the combination of the multiple images if eachindividual measurement uses a similar dynamic range as a single scanningmeasurement.

Turning now to FIG. 8 in an alternative processing technique performedby the processing unit or processor PU, the parameters are calculatedfor each individual image at each measurement position, the results thenbeing combined and used as a basis of the further processing.

Thus, referring to the upper-most boxes in FIG. 8 as in the processdescribed in relation to FIG. 7, each image (1) . . . (n) is correctedbased on the corresponding reference image (1) . . . (n), to provide acorrected image (1) . . . (n). As before this procedure may not benecessary in some circumstances. Parameter calculation for eachcorrected image (1) . . . (n) is then performed separately.

A statistical analysis of the set of results of the images is thenperformed as indicated in the large box marked 1 in FIG. 8, to obtainthe target parameter values including the mean or average and medianvalues. The variation parameters can then also be determined such asvariance, the standard deviation, the minimum and maximum valuesconfidence level, the mode and outlier identification.

Optional processing may then be performed as indicated in the large boxmarked 2 in FIG. 8, in particular data filtering to remove data pointsoutside preset limits. Statistics may then be calculated from the set ofdata defining the images, to produce revised target parameters based onthe statistical information.

It will be appreciated that in this second process the variance of themeasurement results produced by the statistical processing can be usedto determine the spread in the measurement parameter over the target orthe stability of the parameter determination can be determined from themeasurement data. In either case an indication of the reliability andstability of the measurement is produced. Furthermore, flyer removalallows reduction of the uncertainty of the measurement value.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography; itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

Further embodiments are disclosed in the subsequent numbered clauses:

1. A scatterometer configured to measure a property of a substrate,comprising

a radiation source configured to provide a radiation beam to produce aradiation spot on a target on a surface of the substrate, the size ofthe radiation spot being smaller than the target in one direction alongthe target, the position of the radiation spot moving along the surfacein the direction in a series of discrete steps;

a detector configured to detect a spectrum of the radiation beamreflected from the target on the surface of the substrate and to producea measurement signal representative of the spectrum at each position ofthe radiation spot along the direction; and

a processor configured to process the measurement signals produced bythe detector corresponding to each position of the radiation spot toderive a single value for the property.

2. A scatterometer according to clause 1, wherein the processor isconfigured to correct each measurement signal using a respectivereference signal corresponding to the position of the radiation spot onthe target.

3. A scatterometer according to clause 2, wherein the detector isconfigured to measure each reference signal.

4. A scatterometer according to clause 1, wherein the processor isconfigured to combine signals representative of each of the measurementsignals to produce a combined signal and to use the combined signal toderive the single value for the property.

5. A scatterometer according to clause 4, wherein the signals arecombined by, taking the average value of the measurement signals.

6. A scatterometer according to clause 4, wherein the signals arecombined by taking the median value of the measurement signals.

7. A scatterometer according to clause 1, wherein the processor isconfigured to derive a plurality of values for the property from eachsignal indicative of each measurement signal and to combine theplurality of values to produce the derived single value for theproperty.

8. A scatterometer according to clause 7, wherein the processor isconfigured to use the plurality of values to produce statisticalinformation relating to the derived single value.

9. A scatterometer according to clause 8, wherein the statisticalinformation comprises the variance, or the standard deviation, or theminimum and maximum values, or the confidence level, or the mode andoutlier identification, or any combination of the preceding.

10. A scatterometer according to clause 8, wherein the processor isconfigured to use the statistical information to redefine a targetparameter for the value of the property.

11. A scatterometer according to any one of the preceding clauses,wherein the target is a grating structure formed on the surface of thesubstrate, and the measurement signals produced by the detector areprocessed to produce a single value derived from a plurality of lines ofthe grating structure.

12. A scatterometer according to any one of the preceding clauses,wherein the detector is configured to detect an angle resolved spectrumof the radiation beam reflected at a plurality of angles from thetarget.

13. A method of measuring a property of a substrate, comprisingproviding a radiation beam to produce a radiation spot on a target on asurface of the substrate, the size of the radiation spot being smallerthan the target in one direction along the target;

moving the position of the radiation spot along the target in thedirection in a series of discrete steps;

detecting a spectrum of the radiation beam reflected from each positionof the radiation spot along the direction and producing a measurementsignal representative of the spectrum at each position of the radiationspot; and

processing the measurement signals corresponding to each position of theradiation spot to derive a single value for the property.

14. A method according to clause 13, comprising correcting eachmeasurement signal using a respective reference signal corresponding tothe position of the radiation spot on the target.

15. A method according to clause 14, wherein each reference signal ismeasured by the same detector as that used to measure the measurementsignals.

16. A method according to clause 13, wherein the processing includescombining signals representative of each of the measurement signals toproduce a combined signal, the combined signal being used to derive thesingle value for the property.

17. A method according to clause 16, wherein the signals are combined bytaking the average value of the measurement signals.

18. A method according to clause 16, wherein the signals are combined bytaking the median value of the measurement signals.

19. A method according to clause 13, wherein a plurality of values forthe property is derived from each signal indicative of each measurementsignal and combined to produce the derived single value for theproperty.

20. A method according to clause 19, wherein the plurality of values isused to produce statistical information relating to the derived singlevalue.

21. A method according to clause 20, wherein the statistical informationcomprises the variance, or the standard deviation, or the minimum andmaximum values, or the confidence level, or the mode and outlieridentification or any combination of the preceding.

22. A method according to clause 20, wherein the statistical informationis used to redefine a target parameter for the value of the property.

23. A method according to any one of clauses 13 to 22, wherein a gratingstructure is formed on the surface of the substrate, and the measurementsignals produced by the detector are processed to produce a single valuederived from a plurality of lines of the grating structure.

24. A method according to any one of clauses 13 to 23, wherein thedetecting includes detecting an angle resolved spectrum of the radiationbeam reflected at a plurality of angles from the target.

25. A lithographic apparatus comprising:

an illumination optical system arranged to illuminate a pattern;

a projection optical system arranged to project an image of the patternon to a substrate; and

a scatterometer configured to measure a property of the substrate, thescatterometer comprising:

a radiation source configured to provide a radiation beam to produce aradiation spot on a target on a surface of the substrate, the size ofthe radiation spot being smaller than the target in one direction alongthe target, the position of the radiation spot moving along the surfacein the direction in a series of discrete steps;

a detector configured to detect a spectrum of the radiation beamreflected from the target on the surface of the substrate and to producea measurement signal representative of the spectrum at each position ofthe radiation spot along the direction; and a processor configured toprocess the measurement signals produced by the detector correspondingto each position of the radiation spot to derive a single value for theproperty.

26. A lithographic cell comprising:

a coater arranged to coat substrates with a radiation sensitive layer;

a lithographic apparatus arranged to expose images onto the radiationsensitive layer of substrates coated by the coater;

a developer arranged to develop images exposed by the lithographicapparatus; and a scatterometer configured to measure a property of asubstrate, the scatterometer comprising

a radiation source configured to provide a radiation beam to produce aradiation spot on a target on a surface of the substrate, the size ofthe radiation spot being smaller than the target in one direction alongthe target, the position of the radiation spot moving along the surfacein the direction in a series of discrete steps;

a detector configured to detect a spectrum of the radiation beamreflected from a target on the surface of the substrate and to produce ameasurement signal

representative of the spectrum at each position of the radiation spotalong the direction; and

a processor configured to process the measurement signals produced bythe detector corresponding to each position of the radiation spot toderive a single value for the property.

27. A device manufacturing method comprising:

using a lithographic apparatus to form a pattern on a substrate; and

determining a value related to a parameter of the pattern by:

providing a radiation beam to produce a radiation spot on the target,the size of the radiation spot being smaller than the target in onedirection along the target;

moving the position of the radiation spot along the target in thedirection in a series of discrete steps;

detecting a spectrum of the radiation beam reflected from each positionof the radiation spot along the direction and producing a measurementsignal representative of the spectrum at each position of the radiationspot; and

processing the measurement signals corresponding to each position of theradiation spot to derive a single value for the property.

28. A scatterometer configured to measure a property of a target on asubstrate, the scatterometer comprising:

-   -   a radiation source configured to produce a radiated spot on the        target, wherein the radiated spot is moved between discrete        positions along a length of the target;    -   a detector configured to generate measurement signals that        correspond to respective ones of the positions of the radiated        spot; and    -   a processor configured to output, based on the measurement        signals, a single value that is representative of the property        of the target.

29. The scatterometer of clause 28, wherein the radiated spot is smallerthan the target in a dimension along the length of the target.

30. The scatterometer of clause 28, wherein the processor is configuredto:

-   -   correct the measurement signals using respective reference        signals that correspond to the respective positions of the        radiated spot on the target; and    -   derive, based on the corrected measurement signals, the single        value.

31. The scatterometer of clause 30, wherein the detector is furtherconfigured to measure each of the respective reference signals.

32. The scatterometer of clause 31, wherein the detector is configuredto measure the measurement signals and the respective reference signalssimultaneously.

33. The scatterometer of clause 28, wherein the processor is configuredto:

-   -   correct the measurement signals using respective reference        signals that correspond to the respective positions of the        radiated spot on the target;    -   derive values representative of the property of the target at        the respective positions on the target; the values being derived        based on respective ones of the corrected measurement signals;        and    -   derive, based on the values, the single value.

34. The scatterometer of clause 33, wherein the processor is furtherconfigured to use the derived values representative of the property ofthe target at the respective positions on the target to producestatistical information relating to the derived single value, whereinthe statistical information comprises a variance, a standard deviation,minimum and maximum values, a confidence level, or a mode and outlieridentification.

35. The scatterometer of clause 34, wherein the processor is configuredto use the statistical information to redefine a target parameter forthe derived single value of the property.

36. The scatterometer of clause 28, wherein the processor is configuredto:

-   -   combine the measurement signals to output a combined signal; and    -   derive, based on the combined signal, the single value.

37. The scatterometer of clause 28, wherein the processor is configuredto:

-   -   determine an average value of the measurement signals; and    -   derive, based on the average value, the single value.

38. The scatterometer of clause 28, wherein the processor is configuredto:

-   -   determine a median value of the measurement signals; and    -   derive; based on the median value, the single value.

39. The scatterometer of clause 28, wherein the target comprises agrating structure on the surface of the substrate, and the measurementsignals correspond to lines of the grating structure.

40. The scatterometer of clause 28, wherein the detector is configuredto detect an angle resolved spectrum of the radiated spot reflected at aplurality of angles from the target.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365; 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

The descriptions above are intended to be illustrative; not limiting.Thus; it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A scatterometer configured to measure a property of a target on asubstrate, the scatterometer comprising: a radiation source configuredto produce a radiated spot on the target, wherein the scatterometer isconfigured to adjust a position of the radiated spot along a firstdirection across the target and along a second direction that is at anangle with respect to the first direction; a detector configured toreceive radiation scattered by the target, the received radiation beingassociated with positions of the radiation spot on the target along atleast the first direction, and to generate measurement signals based onthe positions of the radiated spot on the target; and a processorconfigured to output, based on the measurement signals, a single valuethat is representative of the property of the target.
 2. Thescatterometer of claim 1, wherein the first direction defines an X axisand the second direction comprises a directional component along the Xaxis and a directional component along a Y axis that is orthogonal tothe X axis.
 3. The scatterometer of claim 1, wherein the radiated spotis smaller than the target in a dimension along the length of thetarget.
 4. The scatterometer of claim 1, wherein the processor isfurther configured to: correct the measurement signals using respectivereference signals that correspond to respective ones of the positions ofthe radiated spot on the target; and derive, based on the correctedmeasurement signals, the single value.
 5. The scatterometer of claim 4,wherein the detector is further configured to measure each of therespective reference signals.
 6. The scatterometer of claim 5, whereinthe detector is further configured to measure the measurement signalsand the respective reference signals simultaneously.
 7. Thescatterometer of claim 1, wherein the processor is further configuredto: correct the measurement signals using respective reference signalsthat correspond to respective ones of the positions of the radiated spoton the target; derive values representative of the property of thetarget at the respective ones of the positions on the target, the valuesbeing derived based on respective ones of the corrected measurementsignals; and derive, based on the values, the single value.
 8. Thescatterometer of claim 7, wherein: the processor is further configuredto use the derived values representative of the property of the targetat the respective positions on the target to produce statisticalinformation relating to the derived single value, and the statisticalinformation comprises a variance, a standard deviation, minimum andmaximum values, a confidence level, or a mode and outlieridentification.
 9. The scatterometer of claim 8, wherein the processoris further configured to use the statistical information to redefine atarget parameter for the derived single value of the property.
 10. Thescatterometer of claim 1, wherein the processor is further configuredto: combine the measurement signals to output a combined signal; andderive, based on the combined signal, the single value.
 11. Thescatterometer of claim 1, wherein the processor is further configuredto: determine an average value of the measurement signals; and derive,based on the average value, the single value.
 12. The scatterometer ofclaim 1, wherein the processor is further configured to: determine amedian value of the measurement signals; and derive, based on the medianvalue; the single value.
 13. The scatterometer of claim 1; wherein thetarget comprises a grating structure on the surface of the substrate,and the measurement signals are based on the lines of the gratingstructure.
 14. The scatterometer of claim 13, wherein the firstdirection is parallel to a direction of a width of the lines.
 15. Thescatterometer of claim 1, wherein the detector is further configured todetect an angle resolved spectrum of the radiated spot reflected at aplurality of angles from the target.
 16. The scatterometer of claim 1,wherein the adjusting the position of the radiated spot is performedsuch that illuminated portions of the target, corresponding torespective ones of the positions of the radiated spot, overlap.
 17. Ascatterometer configured to measure a property of a target on asubstrate, the scatterometer comprising: a radiation source configuredto produce a radiated spot on the target, wherein the scatterometer isconfigured to adjust a position of the radiated spot along a firstdirection across the target and along a second direction that is at anangle with respect to the first direction; a detector configured toreceive radiation scattered by the target and to generate measurementsignals based on positions of the radiated spot on the target; and aprocessor configured to output, based on the measurement signals, asingle value that is representative of the property of the target. 18.The scatterometer of claim 17, wherein the received radiation isassociated with positions of the radiation spot on the target along thefirst direction.
 19. The scatterometer of claim 17, wherein the receivedradiation is associated with positions of the radiation spot on thetarget along the second direction.